F90 module

The functionality of the module unit is similar to that of the C header file.
Also it's like C++ header file defining namespace, therefore, other components, who USE it, can access its content.

A form of a module is as follows

   MODULE name
      specifications
   END MODULE name

See example below:
To make use of the module in the main program, we employ the USE statement. We use a special form of the USE statement that specifies that we are only interested in variable radius in the main program, namely, USE Circle, ONLY : radius. Similarly, we make use of only the parameter Pi in the function subprogram Area_Circle by way of the USE statement appearing in that routine.

eg 1:

MODULE Circle
   REAL, PARAMETER :: Pi = 3.1415927
   REAL :: radius
END MODULE Circle

PROGRAM Area
USE Circle, ONLY : radius     ! only interested in radius. 
IMPLICIT NONE
INTERFACE  ! check function signature 
   FUNCTION Area_Circle (r)
     REAL, INTENT(IN) :: r
   END FUNCTION Area_Circle
END INTERFACE

write(*, '(A)', ADVANCE = "NO") "Enter the radius of the circle:  "
read(*,*) radius    ! radius is NOT defined in main code, included from Module Circle

write(*,100) "Area of circle with radius", radius, " is", &
            Area_Circle(radius)
100 format (A, 2x, F6.2, A, 2x, F11.2)

END PROGRAM Area

FUNCTION Area_Circle(r)
USE Circle, ONLY : Pi      ! only interested in constant/parameter Pi. 
IMPLICIT NONE
REAL :: Area_Circle
REAL, INTENT(IN) :: r

Area_Circle = Pi * r * r    ! Pi is NOT defined in function either, included from module.

END FUNCTION Area_Circle

eg 2:

MODULE Dyn_Array
   INTEGER :: n
   REAL, DIMENSION(:), ALLOCATABLE :: A
END MODULE Dyn_Array

PROGRAM Play_with_Array
USE Dyn_Array
IMPLICIT NONE
INTERFACE   ! checking subroutine signature
   SUBROUTINE Get_Data
   END SUBROUTINE Get_Data
END INTERFACE

INTERFACE  ! checking subroutine signature
   SUBROUTINE Dealloc_Array
   END SUBROUTINE Dealloc_Array
END INTERFACE

REAL :: Prod_A, Sum_A
CALL Get_Data     ! load data into A
Prod_A = PRODUCT(A)    ! use A
write(*,100) "The product of the elements of array A area", &
              Prod_A
Sum_A = SUM(A)         ! use A

write(*,100) "The sum of the elements of array A are", &
             Sum_A

CALL Dealloc_Array        ! unload A
100 format (A, 2x, F11.2)

END PROGRAM Play_with_Array

SUBROUTINE Get_Data
USE Dyn_Array      ! include everything from module (header file in c)
IMPLICIT NONE
INTEGER :: AllocateStatus
write(*,'(A)', ADVANCE = "NO") "Input the number of elements desired:  "
read(*,*) n
ALLOCATE( A(n), STAT = AllocateStatus)    ! load A
IF (AllocateStatus /= 0) STOP "*** Not enough memory ***"

write(*, '(A)', ADVANCE = "NO") "Input array values:  "
read(*,*) A
END SUBROUTINE Get_Data

SUBROUTINE Dealloc_Array
USE Dyn_Array
IMPLICIT NONE
INTEGER :: DeAllocateStatus
DEALLOCATE( A, STAT = DeAllocateStatus)    ! unload A
IF (DeAllocateStatus /= 0) STOP "*** Trouble deallocating ***"
END SUBROUTINE Dealloc_Array